Needle retraction features have been incorporated into syringe assemblies to protect users from needle stick injuries. In conventional assemblies, the needle hub assemblies including a needle cannula, are attached to the syringe barrel and must be withdrawn into the syringe barrel by a user or by a retraction feature. Alternatively, a needle shield may be placed over the needle cannula by the user or otherwise.
In conventional syringe assemblies in which the needle hub assemblies are retracted into the syringe barrel by a retraction feature, the retraction feature is often provided within the syringe barrel and/or the plunger rod disposed within the syringe barrel. Specifically, the plunger rod may include a chamber that houses the needle hub after it is retracted. The retraction feature typically includes a cutting element disposed between the plunger rod and the stopper that is used to open the stopper after the contents of the syringe barrel are expelled, to expose the chamber of the plunger rod to receive the retracted needle hub. A spring is often incorporated in the needle hub assemblies to drive the retraction of the needle hub into the plunger rod.
Accordingly, such retraction features require cutting, braking, piercing or other force-intensive mechanical action for activation and, thus, increased complexity to enable the sealed plunger and stopper to be breached during activation. Further, as most conventionally designed retractable needles are activated after dosing by continued pressure on the rear of the plunger rod, inadvertent activation of the retraction feature may occur since the same forces must be applied when expelling the contents of the syringe barrel. Moreover, some devices may be inadvertently activated during dosing if sufficient pressure is generated during expulsion of the contents of the syringe, for example, when the medication is viscous and requires the user to apply additional pressure or force to the plunger rod, which exceeds the force required to activate the retraction feature. Premature activation is especially problematic in applications where high forces are applied to the plunge rod, for example, during high speed injections.
The retraction features of conventional syringe barrels that are plunger-activated must withstand increased syringe pressures and associated increase in force applied to the plunger rod, as described above. These increased forces and pressure leads to a requirement for large activation forces which could exceed the operational forces in order to prevent premature activation. Since most conventional syringe barrels employ an additional plunger motion after full dispensing, and in the same manner as the dispensing motion, a threshold force must be used to allow the user to differentiate between a fully bottomed plunger and the activation of the retraction feature. The threshold force may be difficult to ascertain and maintain separately from the force applied to the plunger rod to expel the contents of the syringe barrel. Further, proper application of the threshold force may require a user to position the syringe barrel and the needle cannula at an increased angle to the patient's skin, instead of positioning the syringe barrel and needle cannula substantially parallel to the patient's skin. The additional force required to activate the retraction feature may cause additional pressure to be generated at the stopper or other removable opening in the plunger rod, which may be sufficient to cause the stopper and/or plunger rod to malfunction.
In conventional syringe assemblies where the retraction feature and the subsequent housing of the needle hub in conventional retractable syringes are contained within the fluid path, the retraction feature and housing may cause a volume of medication to become trapped within the syringe barrel, thereby increasing waste and potentially affecting dosing accuracy. Further, a portion of the trapped medication may be expelled during the activation of the retraction feature causing splatter, if the retraction feature is activated when the needle cannula is outside of the patient, or an unintended increase in the dose administered to the patient, if the retraction feature is activated when the needle cannula is in the patient. Placement of the retraction feature within the syringe barrel may also cause trapped air to remain in the syringe barrel when purging or priming the syringe. This can lead the possibility of injected air. The size of the syringe barrel must also accommodate the retraction feature and the needle hub assembly that will be housed therein after retraction.
In syringe assemblies which do not house the retraction feature within the fluid path, the retraction feature is often disposed at a location that requires the user to change their grip of the syringe assembly to activate the retraction feature.
Conventional retraction syringe assemblies often do not incorporate reuse prevention features, and thus, the retraction mechanism may be reset so the syringe barrel may be reused. The reuse of syringe assemblies without sterilization or sufficient sterilization is believed to facilitate the transfer of contagious diseases. Further, the retraction features of conventional syringes also often require the user to actively activate the retraction mechanism. Accordingly, the chance of human error in failure to activate or properly activate the retraction mechanism can lead to continued exposure of needles.
Accordingly, it would be desirable to provide a retractable syringe assembly with a retraction feature that does not interfere with normal operation of the syringe assembly, is passively activated and reduces the risk of premature activation or the retraction mechanism. It would also be desirable to provide a retractable syringe assembly which incorporates a reuse prevention feature.